for Web Applications 01 Introduction to Perl Alexandros Labrinidis - PDF document

CS 1520 / Fall 2012 Programming Languages for Web Applications 01 – Introduction to Perl Alexandros Labrinidis University of Pittsburgh What is Perl n P ractical E xtraction and R eporting L anguage n 1987 n Larry Wall n roots in unix applications (awk/sed) n http://history.perl.org n Uses of Perl: n EVERYWHERE :-) n Scripts for repeated tasks n Powerful regular expression matching n Text processing n Web programming 2 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 1

How to run Perl programs (revised) Use text editor (say pico) to write and save program, say hello.pl n n Option 1: perl -w hello.pl Next time: n perl - w hello.pl n Option 2: n Make sure first line of hello.pl is the following #!/bin/perl -w chmod u+x hello.pl ./hello.pl Next time: n ./hello.pl 3 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � Hello World #!/bin/perl -w print ( “ Hello world!\n ” ); n Observations: n First line is needed to make program self-executable n -w flag is highly recommended: it prints out warnings n All statements in in semicolon ; n Strings are enclosed in double quotes “ (more on this later) 4 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 2

Scalar Data: Numbers When we have “ one of something ” n n E.g., numbers, strings Examples:Floating point literals Examples: Integer literals n n n 1.25 n 0 n 255.000 n 2001 n 255.0 n 1520 n 3.14159 n -40 n -6.5e24 n 61298040283768 n 1.2E-4 n 61_298_040_283_768 5 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � Scalar Data: Numbers n Examples: non-decimal integer n 0377 is 377 octal, same as 255 decimal n 0xff is FF hex, same as 255 decimal n 0b11111111 is in binary, same as 255 decimal n 0x50_65_72_7C 6 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 3

Numeric operators n 2 + 3 # 2 plus 3, or 5 n 5.1 - 2.4 # 5.1 minus 2.4, or 2.7 n 3 * 12 # 3 times 12 = 36 n 14 / 2 # 14 divided by 2, or 7 n 10.2 / 0.3 # 10.2 divided by 0.3, or 34 n 10 / 3 # always floating-point divide, so 3.3333333... n Modulus operator: n 10 % 3 # 10 module 3, or 1 n 10.5 % 3.2 # first converted to 10 % 3 n Exponent operator: n 2 ** 3 # 2 to the 3rd, or 8 7 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � Scalar Data: Strings n Strings are sequences of characters (like hello). n Strings may contain any combination of any characters n Two types: n Single-quoted string literals n No variable substitution in string n Double-quoted string literals n Allow for variable substitution 8 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 4

Single-quoted Strings n 'fred' # those four characters: f, r, e, and d n 'barney' # those six characters n '' # the null string (no characters) n 'Don\'t let an apostrophe end this string prematurely! ’ n 'the last character of this string is a backslash: \\ ’ n 'hello\n' # hello followed by backslash followed by n n 'hello there' # hello, newline, there (11 characters total) n ’ \ ’ \\' # single quote followed by backslash Note that the \n within a single-quoted string is not interpreted as a newline, n but as the two characters backslash and n. Only when the backslash is followed by another backslash or a single quote does it have special meaning. 9 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � Double-quoted strings n "barney" # just the same as 'barney ’ n "hello world\n" # hello world, and a newline n "The last character of this string is a quote mark: \" ” n "coke\tsprite" # coke, a tab, and sprite 10 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 5

String operators n Concatenation n "hello" . "world" # same as "helloworld ” n "hello" . ' ' . "world" # same as 'hello world ’ n 'hello world' . ” \n" # same as "hello world\n ” n String repetition n "fred" x 3 # is "fredfredfred ” n "barney" x (4+1) # is "barney" x 5, or "barneybarneybarneybarneybarney ” n 5 x 4 # is really "5" x 4, which is "5555" 11 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � Scalar Variables n A variable is a name for a container that holds one or more values n Unlike other languages, there is no variable definition/declaration in Perl (unless you invoke “ strict ” ) n Example variable names: n $a_very_long_variable_that_ends_in_1 n $a_very_long_variable_that_ends_in_2 n $Fred is different from $fred n $is_it_better_with_underscores n $orIsItBetterWithCaps 12 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 6

Assignment n $fred = 17; # give $fred the value of 17 n $barney = 'hello'; # give $barney the five-character string 'hello ’ n $barney = $fred + 3; # give $barney the current value of $fred plus 3 (20) n $barney = $barney * 2; # $barney is now $barney multiplied by 2 (40) 13 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � Binary Assignment n $fred = $fred + 5; # without the binary assignment operator n $fred += 5; # with the binary assignment operator n $barney = $barney * 3; $barney *= 3; n $str = $str . " "; # append a space to $str $str .= " "; # same thing with assignment operator 14 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 7

Print print "hello world\n"; � # say hello world, followed by a newline � print "The answer is "; � print 6 * 7; � print ".\n"; � � OR: � print "The answer is ", 6 * 7, ".\n"; 15 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � Scalar Variables into strings n $meal = "brontosaurus steak"; n $barney = "fred ate a $meal"; # $barney is now "fred ate a brontosaurus steak ” n $barney = 'fred ate a ' . $meal; # another way to write that 16 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 8

Lists and Arrays in Perl n Q: What is a List or Array? n A: a list is ordered scalar data n A: an array is a variable that holds a list n Example - list literals: n (1, 2, 3) #array of three values: 1, 2, and 3 n ( “ fred ” , 4, 5) #array of three values: “ fred ” , 4, and 5 n ($a, 42) #two values: current value of $a and 42 n ($a+$b, $c+$d) #also two values n () #the empty list (no elements) n (1 .. 5) #same as (1, 2, 3, 4, 5) 17 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � More examples - arrays n @a = ( “ rachel ” , “ phoebe ” , “ chandler ” , “ ross ” , “ monica ” , “ joey ” ); n Shortcut: n @a = qw (rachel phoebe chandler ross monica joey); n qw stands for quote word : it creates a list from the nonwhitespace parts within the parentheses n @b = qw(mary 2 5 had a little 4 lamb 6); n Q: what will the following do? print ( “ the answer is “ ,@a, ” \n ” ); 18 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 9

Arrays - Assignment n @rachel = (1, 2, 3); #the rachel array is a 3-element list literal n @monica = @rachel; #the rachel array is copied to the monica array n @joey = 1; # 1 is automatically converted to (1) n @ross = qw(one two three); # equivalent to @ross = ( “ one ” , “ two ” , “ three ” ); 19 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � Arrays - Assignment (RHS) n @phoebe = qw(smelly cat); #equivalent to @phoebe = ( “ smelly ” , “ cat ” ); n @chandler = ( “ here ” , “ is ” , @phoebe, 5, 6); #chandler becomes ( “ here ” , “ is ” , “ smelly ” , “ cat ” , 5, 6); n @chandler = (42, @chandler); #puts 42 in front of chandler n @chandler = (@chandler, “ last ” ); #puts “ last ” at the end of chandler 20 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 10

Arrays - Assignment (LHS) n ($a, $b, $c) = (1, 2, 3) #give value of 1 to $a, 2 to $b, 3 to $c n ($a, $b) = ($b, $a) #swap $a and $b n ($d, @ross) = ($a, $b, $c) #give $a to $d, and ($b, $c) to @ross n ($f, @ross) = @ross; #remove first element of @ross and give it to $f #this makes $f = $b, and @ross = ($c) 21 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � Scalar and List context n Perl will do automatic “ conversion ” of an array, depending on the context. n List context : normal n Scalar context : when the array must be “ squeezed ” into a scalar variable. n In this case, the length of the array is returned! n Examples: n @barney = @fred; # all of @fred is copied to @barney n ($a) = @fred; # $a gets the first element of @fred n $a = @fred; # $a gets the length of @fred 22 � Alexandros Labrinidis, Univ. of Pittsburgh � CS 1520 / Fall 2012 � 11